This invention relates to servo heads in disc file data storage systems. In particular, the present invention relates to improved passive noise cancellation in servo heads by the use of a servo transducer and an integral noise cancellation coil.
Disc stack data storage systems contain a spindle which supports a number of magnetic media discs on which data is stored in concentric tracks by means of data heads adjacent to the discs which write and read magnetically encoded data. These data heads are mechanically linked to a servo head located on an electro mechanical actuator. The servo head and the actuator are part of a negative feedback, closed loop servo system which is used to move the data heads (and the servo head) from one track to another on their respective discs for track seek operations and to hold them directly over a track for track following.
The normal output signal of the servo head is encoded radial position information, which it reads from the servo disc. However, the servo head is also susceptible to picking up external electromagnetic radiation. Although all of the data heads in the disc stack data storage system can cause external electromagnetic radiation, in many instances, the main source of this external electromagnetic radiation is the data head adjacent to the data disc located across from the servo head and disc on the spindle. When that data head is writing data, it is also radiating noise to the servo head. The noise corrupts the normal radial position information for the servo system, thus, reducing the track following precision. Consequently, the data written at that time may not be completely within the actual data track boundaries. Later, when this data is being read, the normal track following precision has returned. However, the data head is then flying within the track boundaries trying to read data which may not be written completely within the track boundaries. This increases the probability that data errors will occur during reading.
A passive noise cancellation technique for servo heads is described in a commonly-assigned patent entitled Thin Film Servo Head Employing The Inactive Transducer Coils To Cancel Write Noise From Nearby Data Heads by L. A. Wilcox, U.S. Pat. No. 4,962,437, issued Oct. 9, 1990, and which is hereby incorporated by reference. In this patent, the servo head includes two servo transducers. The first servo transducer reads the servo information stored on the servo disc. The second servo transducer is located on the same slider as the first servo transducer, but has its pole tips recessed slightly from the servo disc surface to prevent it from reading a signal from the servo disc. Also, the two servo transducers are located on opposite rails or skis of the servo slider. The second servo transducer is connected to the first servo transducer with opposite coil polarity in either a series-opposing or parallel-opposing configuration. The noise induced in the second servo transducer (which is generated by the adjacent data head when it is writing data) tends to cancel similar noise which is induced in the first servo transducer. Therefore, during normal operation, the net output voltage of this combined servo head, even in the presence of noise, more closely approximates the signal read from the servo disc by the first servo transducer.
An impediment to the effectiveness of passive noise cancellation techniques for servo heads is the effect of signals from the servo disc on the second servo transducer. Although the second servo transducer may have its pole tips recessed slightly to reduce the effects of a signal from the servo disc, the effects can not always be totally eliminated. The poles of the second servo transducer may still link some signals from the servo disc to the second transducer's coil. Additionally, servo heads with two servo transducers may require more process steps to fabricate than single transducer servo heads. This is particularly true when one servo transducer is fabricated in close proximity to the other. For instance, if one servo transducer is placed on top of the other servo transducer, the number of process steps may be more than doubled. Each additional process step results in increased cost and complexity of the fabrication process.